1. Field of the Invention
The present invention relates to a light guide and stamper production method, particularly to a method for producing a light guide and a stamper, with the light guide to be used in a back light module of a liquid crystal display (LCD) panel.
2. Description of Related Art
Flat displays which do not emit light, like liquid crystal display (LCD) panels, are illuminated by flat light module. A light transmitting LCD panel is illuminated by a back light module, whereas a light reflecting LCD panel is illuminated by a front light module. The back light module has a linear light source, e.g. a cold cathode tube, or a point-like light source, e.g. a light emitting diode (LED), forming a simple lighting assembly of high intensity and uniform spatial intensity distribution. As shown in FIGS. 3A and 3B, a conventional back light module has a cold cathode tube 1, which is attached to one end of a light guide 2. Light from the cold cathode tube 1 is by a reflector 1A reflected onto the light guide 2. Inside the light guide 2, primary light propagates along a main direction, parallel to front and back surfaces of the light guide 2, with little attenuation and, due to total reflection, without exiting through the front and back surfaces. Microstructures 3 that are densely arranged on the back surface of the light guide 2 when hit by rays of the primary light deflect these as secondary light towards the front surface. The secondary light is no longer totally reflected, but rather passes through the front surface. This decreases the intensity of the primary light within the light guide not yet deflected by microstructures 3. For this reason, the microstructures 3 are arranged close to the light source 1 at a low a real density and, proceeding away from the light source 1, with increasing a real density, so that the secondary light emanating front the front surface of the light guide 2 has a uniform spatial intensity distribution. A rear diffusion sheet 4 covers the front surface of the light guide 2 spreads the secondary light to a widened range of emission angles and smoothes out any image of the microstructures 3. A rear prism sheet 5 and a front prism sheet 6 are laid on the rear diffusion sheet 4. The rear prism sheet 5 and the front prism sheet 6 are transparent plates, each carrying parallel rows of V-shaped projections, with the V-shaped projections of the rear prism sheet 5 oriented perpendicular to the V-shaped projections of the front prism sheet 6. Thereby the angular distribution of the secondary light is concentrated around a direction normal to the front surface, increasing intensity and observation angle. Finally, a front diffusion sheet is laid on the front prism sheet 6, smoothing out images of the rear and front prism sheets 5, 6.
The microstructures 3 on the back surface of the light guide 2 have quadratic, bowl like, oval or semicircular shapes for adjusting uniformity of the secondary light and have sizes ranging from centimeters to microns. The smaller the sizes of the microstructures 3, the more accurately uniformity of the secondary light is adjustable, so that impairing of imaging quality is avoided. Other embodiments of the light guide 2 are possible. For instance, the front surface of the light guide 2 alternatively has mirrors, or, in another variation has microstructures which are V-shaped grooves substituted for one of the prism sheets, so that a combined optical structure is formed, having the same optical characteristics as the conventional front surface of the light guide 2 and the front prism sheet 6, allowing to dispense with one of the prism sheets. Alternatively, pyramid like microstructures are set on the front surface of the light guide 2, having the same optical characteristics as the conventional front surface of the light guide 2 and the rear and front prism sheets 5, 6, allowing to dispense with both of the prism sheets 5, 6. However, in both embodiments, inclinations and angles have to be set precisely, so that high variability and a high standard of size precision of the microstructures are required.
Microstructures on conventional light guides are manufactured by the following methods: 1. Matrix printing, putting dye or resin on the surface of the light guide. This method, however, is limited to matrix sizes of more than 300 microns, and stability and variability of shapes of microstructures are insufficient. 2. Sand-blasting and etching, using a metal plate as a mold, forming the light guide by electroforming, and injection molding or hot embossing. This does not provide for well-defined shapes of microstructures, nor is there any variability of microstructures. 3. Mechanical working, making a mold using a diamond cutting tool, making a stamper by electroforming, then forming the light guide by plastic injection molding or hot embossing using the stamper. Although this method allows to control size and shape of the microstructures, there is no variability of microstructures. Due to the size of the cutting tool, microstructures cannot be made smaller than several tens of microns. Inaccuracy of mechanical working as well as wear on the cutting tool results in shapes of microstructures that are not precisely reproduced. 4. Photoresist light-engraving, as taught in U.S. Pat. No. 5, 776, 636. Coating with a photoresist layer, exposure to light and developing, so that the microstructures are fixed, then making a stamper by electroforming, and producing the light guide by plastic injection molding or hot embossing using the stamper.
Since the shape and size of the microstructures determine optical characteristics thereof, demand on precision is high. Therefore, during a development stage repeated tests are required to obtain a satisfying effect. However, since conventional methods all use a mold, testing is possible only after injection molding or hot embossing. Thus, when a light guide is developed, using the methods described above, a sample has to be produced and then optically tested. This has to be repeated, until the tests are successful. Furthermore, the precision of heights of the microstructures on the light guide depends on the thickness of the photoresist layer, affecting precision of size of the microstructures on the light guide. There is also a limited variability of the microstructures on the light guide.
There are two ways of etching of microstructures, isotropic etching and anisotropic etching. If etching proceeds with equal speeds in all directions (x-, y-, z-axis), isotropic etching is done. By varying components of a liquid etching agent, temperature and stirring, isotropic etching produces various shapes of microstructures, like quadratic, bowl like, oval or semicircular shapes.
Anisotropic etching proceeds with different velocities in anisotropic structures of single crystal materials, thus creating certain etched shapes, e.g. V-shaped, U-shaped or pyramid like microstructures. For performing anisotropic etching, the substrate is a single crystal, like silicon, quartz, GaAs or LiNbO3. Growing of a mask thin film, coating by a photoresist layer, light exposure and etching masked by the mask thin film are performed. Due to the single-crystal structure of the substrate, etching proceeds anisotropically, and certain shapes are possible, like V-shaped, U-shaped or pyramid like microstructures. For example, a less than 100 greater than  silicon wafer is used as a substrate, a SiO2 layer is deposited thereon, a photoresist pattern is coated, exposured, developed thereon, then by HF etching the SiO2 layer is etched and the photoresist layer is removed. The remaining SiO2 pattern serves as a mask for anisotropic etching. Anisotropic etching is then performed for silicon using liquid KOH, NaOH, ethylene-diamine pyrocatechol or N2H4. Due to the single-crystal structure of the substrate, etching proceeds anisotropically, and certain facets are etched away, forming V-shaped, U-shaped or pyramid like microstructures.
The main object of the present invention is to provide a novel light guide and stamper production method which mainly uses light exposure and combined anisotropic and isotropic etching for forming microstructures on a substrate and producing light guides by electroforming. A front stamper for front surfaces of the light guides is made by anisotropic etching, having V-shaped, U-shaped or pyramid like microstructures. Having made the front stamper, the front surfaces of the light guides are readily formed, having V-shaped, U-shaped or pyramid like microstructures, which have well-defined inclinations, so that the front surfaces are brighter and emit light at controlled angles. A rear stamper for back surfaces of the light guides is made by isotropic etching, for forming microstructures with quadratic, bowl like, oval or semicircular shapes. Having made the rear stamper, the back surfaces of the light guides are readily formed, having quadratic, bowl like, oval or semicircular shapes and deflecting light, while being arranged at varying densities, so as to achieve uniform intensity of deflected light, which in turn results in light being emitted uniformly by the front surface. By combining the front and rear stampers for forming the light guides, the back surfaces thereof are shaped like the rear stamper, having quadratic, bowl like, oval or semicircular microstructures, whereas the front surfaces thereof are shaped like the front stamper, having V-shaped, U-shaped or pyramid like microstructures. If the front surfaces of the light guides have V-shaped microstructures, the shape thereof is the same as the shape of conventional planar front surfaces with one prism sheet added, and one prism sheet can be dispensed with. If the front surfaces of the light guides have pyramid like microstructures, the shape thereof is the same as the shape of conventional planar front surfaces with two prism sheets added, and two prism sheets can be dispensed with. This leads to a smaller number of structural parts, resulting in higher quality and lower cost.
Another object of the present invention is to provide a novel testing method for developing light guides which uses combined anisotropic and isotropic etching of a single-crystal, transparent substrate, wherein the front surface thereof is anisotropically etched, forming V-shaped, U-shaped or pyramid like microstructures, and the back surface thereof is isotropically etched, forming quadratic, bowl like, oval or semicircular microstructures. After etching of the transparent substrate, by plastic injection molding a transparent body with the same microstructures is formed, simulating a back light module equipped with a light guide, a reflection sheet, a diffusion sheet or a prism sheet. This allows to conduct tests of light intensity and uniformity without having to make a stamper and molding a light guide. Having finished the optical tests, the stamper is electroformed and light guides are molded. This saves the effort of making light guides by printing, etching or molding from a stamper before optical testing, thus saving time and cost.
The present invention can be more fully understood by reference to the following description and accompanying drawings.